Breakdown of the blood-retinal barrier (BRB) leading to macular edema is a major cause of visual loss in several ocular disorders, but in many, little is known as to how and where the BRB is disrupted. We have demonstrated that these questions can be addressed by immunolocalization of extravascular albumin and that this technique has important advantages including elimination of the need for foreign tracer substances, the ability to perform it on paraffin-embedded sections from human eyes, and the ability to perform it at the electron microscopic (EM) level which allows investigation of mechanisms of BRB breakdown. We will use light microscopic immunohistochemical staining for albumin in human pathologic specimens to help localize the site of BRB breakdown in disorders in which this information is unknown, including aphakic or pseudophakic macular edema, macular edema occurring after other types of ocular surgery, ocular inflammatory diseases, retinitis pigmentosa, and choroidal melanomas. Two disease processes associated with serous macular detachment, central serous retinopathy and pigment epithelial detachments, will also be studied. The mechanism by which BRB breakdown occurs in diabetes will be investigated by performing EM immunocytochemical localization of albumin in human diabetic eyes and in two animal models of diabetes, spontaneously diabetic BB rats and galactosemic rats. Diabetic and galactosemic rats treated with Sorbinil, an aldose reductase (AR) inhibitor, will be concurrently examined to investigate the purported role of enhanced AR activity in diabetes-induced BRB dysfunction. Double labeling for albumin and AR at the EM level in humans and rats will determine whether the cells expressing AR are the same as those that have lost their barrier function. It is likely that BRB breakdown in some ocular disorders chemically mediated. Several agents will be examined for the location and mechanism of their effect on the BRB, by performing intravitreous injections in pigs followed by immunolocalization of albumin at the light and EM levels. These experiments could provide new insights concerning the BRB and how it might be modulated, which could serve as a basis of the design of new treatment approaches for macular edema.